A head mounted display, and in particular a Helmet Mounted Display (HMD) allows a user to view a generated image superimposed on a scenery image. In a typical HMD, this is achieved by projecting the generated image on a transparent surface (i.e., an optical combiner) positioned in front of the user's eyes. The brightness of the generated image should provide a minimum level of contrast relative to the brightness of the scenery, in order for the user to have a good view of both. The visor of a helmet with a HMD may be a tinted visor (i.e., a shaded visor), having lower light transmission than a clear visor. Such tinted visor reduces the brightness of the ambient light arriving at the user's eye, in order to provide the user with a better view in bright conditions. The degree of transmission or alternatively the transmission of certain wavelengths may vary from visor to visor. Furthermore, the transmission of the visor may be adjustable and controllable, automatically or manually. HMD systems with a tinted visor are known in the art.
In HMD systems which include a display located outside of a tinted visor, the display is required to generate a bright-enough image, which, after being attenuated by the tinted visor, would reach the user's eye with similar brightness as the ambient scene image. Therefore, the image generator is required to be a high-power image generator, which requires, respectively, a strong power supply. Such a strong power source would generally consume a high volume of space, which would increase the volume of the display system, and the bulkiness thereof. Furthermore, a high-power image generator and a strong power supply are likely to produce heat, such that would require addition of cooling elements to the display system. Such cooling elements would further increase the volume of the display system.
U.S. Pat. No. 6,008,946 issued to Knowles, entitled “Ambient light display illumination for a head mounted display” is directed to a head mounted display which uses ambient light to backlight a display to similar brightness as the scene being viewed. The system is mounted on a helmet unit or other head-mounting. The system includes a visor, an ambient light director, a display, a beam splitter and a combiner. The beam splitter and the combiner are placed in the line of sight of the user between the user's eye and the visor. The display is situated so that the generated image falls on to the beam splitter. The ambient light director is located so it would direct light to the backside of the display. The visor includes a shaded portion corresponding to the field of view of the user, and a clear portion which allows a predetermined amount of light into a selected portion of the helmet unit. Images from objects in the field of view of the user pass through the shaded portion of the visor, the combiner and through the beam splitter to the user's eye. Ambient light passes through the clear portion of the visor to the ambient light director which directs the light onto the back of the display. The display can be of any type that uses backlighting such as an LCD display. The image generated by the display is reflected by the beam splitter toward the combiner and then toward the user's eye.
U.S. Pat. No. 5,640,711 issued to Lefort et al. entitled “Head-mounted visor with variable transmission”, is directed to a head-mounted visor having at least two zones with different values of absorption of light. The visor includes two functionally different zones. One zone has a higher absorption, and corresponds to the wearer's visual field of the outside scene. Another has a lower absorption, and corresponds to the view of the dashboard instruments.
U.S. Pat. No. 5,835,277 issued to Hegg, entitled “Modular helmet mounted display” is directed to a helmet mounted display system. The system includes a helmet mounted visor/combiner, an image source and relay optics (ISRO). The visor/combiner has a transparent window portion at one side adjacent to one eye of the user, and a transparent viewing portion at the opposite side in front of the other eye of the user. The ISRO projects an image inwardly of the visor/combiner through the window portion and onto the inner surface of the viewing portion of the visor/combiner, focused at infinity. The image is partially reflected from the inner surface of the viewing portion toward the eye of the user, allowing the user to see the image superimposed upon the outside scene.
U.S. Pat. No. 7,893,890 issued to Kelly et al. and entitled “Electrically dimmable combiner optics for head-up display”, is directed to a system for providing head-up displays with variable light transmission. The system includes a combiner and a projector, wherein the projector projects an image onto the combiner. Alternatively, the combiner may have an internally integrated display module. The system also includes a light sensor, which detects the light intensity outside the cockpit portion. The light sensor transfers this information to an operatively coupled combiner control device coupled with the combiner. The control device adjusts the transmission of the combiner after receiving information relating to ambient the light intensity. Further alternatively, the operator may manually control the transmission of the combiner.
The combiner may include one or more segments to provide varying levels of transmission of light. For example, the combiner may be divided vertically, horizontally, or both, to create segments of the combiner. Additionally, the combiner may be wearable, for example in the form of a visor attached to a helmet.